The present invention relates to non-asbestos friction materials made by molding and curing compositions comprising a fibrous base, an inorganic filler, an organic filler and a binder. More specifically, it relates to non-asbestos friction materials which can prevent undesirable jerkiness in low-speed braking, and are thus highly suitable as friction materials for use in large vehicles such as buses and trucks.
A sharp increase in braking performance or effectiveness at low speed, especially in large vehicles such as buses and trucks, results in jerky low-speed braking characterized by sudden stopping of the vehicle accompanied by large, lurching movements. This makes for an uncomfortable ride, and sometimes even causes passenger injuries due to falls inside the vehicle. Hence, there exists a need for a way to prevent such jerkiness in low-speed braking.
At the same time, regulatory in Japan has led to an increased demand for higher braking effectiveness. The following improvements in friction materials have been proposed as ways to achieve better effectiveness.
(1) Add a large amount of metal powder to the friction material.
(2) Include a large amount of glass fibers in the friction material (e.g., at least 10% by volume, based on the overall composition).
(3) Increase the average particle size of the abrasive used in the friction material and suitably adjust the content of the abrasive. A typical example is the use of zirconium silicate or magnesium oxide having an average particle size of at least 10 xcexcm.
However, each of these prior-art methods for increasing braking effectiveness has a number of associated problems. For instance, the first approach, according to which a large amount of metal powder is added to the friction material, causes xe2x80x9cmetal catch,xe2x80x9d resulting in such undesirable effects as scoring of the brake drum and uneven braking action which causes the vehicle to pull to one side during braking.
The second approach, which involves adding at least 10% by weight of glass fibers to the friction material, does improve braking effectiveness. However, this advantage is offset by an increase in the xe2x80x9cspeed spread,xe2x80x9d defined herein as the absolute value of the difference between effectiveness at 50 km/h and effectiveness at 100 km/h, and also larger and undesirable changes over time in both the braking effectiveness and the speed spread.
In the third approach mentioned above, an abrasive such as zirconium silicate or magnesium oxide having an average particle size of at least 10 xcexcm is included in the friction material. This solution is indeed effective for enhancing braking effectiveness, yet it too has a number of shortcomings. Undesirable results include a larger speed spread, decreased effectiveness at high speed, and large and undesirable changes over time in both the speed spread and braking effectiveness. In addition, the abrasive scores the brake drum and is a cause of jerkiness during low-speed braking.
Moreover, owing to the large content of the above added components, these friction materials (1) to (3) cause a higher than necessary degree of wear on the mating surface (e.g., drum or disc surface), shortening the brake life.
Hence, prior-art friction materials all have significant drawbacks. Not only do they fall short of the requirements for such materials, they are unable to prevent undesirable jerkiness during low-speed braking.
It is therefore an object of the present invention to provide a non-asbestos friction material which has a high braking effectiveness in ordinary use, a small speed spread, and minimal change over time in both the braking effectiveness and speed spread, does not give rise to a morning effect, and can prevent jerky movement during low-speed braking. xe2x80x9cMorning effect,xe2x80x9d as used herein, refers to an increase in the braking effectiveness from the initial effectiveness during cold-temperature operation.
We have found that, unlike earlier attempts to enhance braking effectiveness, adding to the friction material composition a specific amount of an inorganic filler having a high hardness and small particle size,.and preferably adding also a smaller than customary amount of chopped glass strand has the surprising and unanticipated effect of providing an outstanding non-asbestos friction material which is endowed with a good braking effectiveness under normal use yet does not cause undesirable jerkiness during low-speed braking.
That is, we have discovered that the incorporation, in a non-asbestos friction material made by molding and curing a composition comprising a fibrous base, an inorganic filler, an organic filler and a binder, of 0.1 to 10% by volume, based on the overall composition, of an inorganic filler having a 90% particle size of 0.1 to 8 xcexcm (as opposed to the particle size of at least 10 xcexcm typical of the prior art) and a Mohs hardness of 6 to 8, and preferably the further incorporation of the lower than conventional amount of 1 to 6% by volume of chopped glass strand, based on the overall composition, elicits synergistic effects between these constituents and other constituents of the friction material. By virtue of these effects, there can be obtained outstanding non-asbestos friction materials which have a high braking effectiveness in normal use (generally about 50 km/h), a small speed spread, and an undiminished braking effectiveness at high speeds, undergo minimal change over time in effectiveness and speed spread, do not give rise to a morning effect, can prevent jerkiness during low-speed braking, cause minimal drum surface roughness and drum wear depth following continuous use, and have outstanding durability and a longer service life.
The reasons for the excellent properties of the inventive friction material are not well understood. However, a likely explanation is that molding the friction material composition in a state where the inorganic filler having a high hardness and a specific 90% particle size is uniformly blended with preferably a small amount of chopped glass strand allows each ingredient to exhibit its full capabilities. This makes it possible to achieve a friction material which, unlike prior-art friction materials that contain only an abrasive having a large average particle size or have a high glass fiber content, has a high braking effectiveness in normal use, and also has a small speed spread, does not undergo a decline in braking effectiveness at high speeds, experiences minimal change in the effectiveness and speed spread over time, can prevent undesirable jerkiness during low-speed braking, causes minimal drum surface roughness and drum wear depth following continuous use, and has excellent durability and a longer service life.
Accordingly, the present invention provides a non-asbestos friction material made by molding and curing a composition comprising a fibrous base, an inorganic filler, an organic filler and a binder, wherein the inorganic filler has a 90% particle size of 0.1 to 8 xcexcm, a Mohs hardness of 6 to 8, and accounts for 0.1 to 10% by volume of the overall composition.
The invention additionally provides a non-asbestos friction material made by molding and curing a composition comprising a fibrous base, an inorganic filler, an organic filler and a binder, which friction material has a difference ratio between the braking effectiveness at 5 km/h (TP1) and the braking effectiveness at 30 km/h (TP2), as determined by low-temperature low-speed braking tests in accordance with Japan Automobile Technology Association standard JASO C407-87 and expressed as (TP1xe2x88x92TP2)/TP1xc3x97100, of at most 40%.
The non-asbestos friction material of the invention can be made by molding and curing a composition composed primarily of a fibrous base, an inorganic filler, an organic filler and a binder. However, to achieve the objects of the invention, it is critical that, of these components, the amount and type of inorganic filler be selected such that a specific proportion of an inorganic filler having a specific Mohs hardness and a specific 90% particle size is formulated within the composition. Moreover, it is recommended that the composition include a lower than conventional amount of chopped glass strand as the fibrous base.
The inorganic filler must have a 90% particle size of 0.1 to 8 xcexcm and a Mohs hardness of 6 to 8, and must account for 0.1 to 10% by volume of the overall friction material composition. The 90% particle size is preferably 0.3 to 6 xcexcm, more preferably 0.5 to 4 xcexcm, and most preferably 0.5 to 3 xcexcm. The Mohs hardness is preferably 7 to 8. The content of inorganic filler is preferably 3 to 9% by volume and most preferably 4 to 8% by volume, based on the overall composition. A friction material made of a composition in which the Inorganic filler has a 90% particle size, a Mohs hardness or a content below these respective ranges is unable to achieve an increased braking effectiveness. On the other hand, the use of inorganic filler having a 90% particle size, a Mohs hardness or a content greater than the above respective ranges results in a friction material that causes jerkiness during low-speed braking. The objects and advantages of the invention cannot be achieved in either of these cases.
The term xe2x80x9c90% particle size,xe2x80x9d as used herein, refers to the particle size at 90% of the cumulative particle size distribution.
The shape of the inorganic filler particles is not critical although a spherical or nearly spherical shape is preferred. The particles may be surface treated if necessary.
Illustrative examples of such inorganic fillers having an average particle size and a Mohs hardness within the indicated ranges include magnesia, zirconium oxide, zirconium sulfide, zirconium silicate, xcex1-quartz (Mohs hardness, 7) and chromium oxide. These may be used alone or as combinations of two or more thereof. The preferred inorganic filler is zirconium silicate (Mohs hardness, 7.5).
In addition to inorganic filler having the above-indicated average particle size and Mohs hardness, the friction material of the invention may contain also other inorganic fillers commonly used in friction materials. Illustrative examples include molybdenum disulfide, calcium carbonate, barium sulfate, calcium hydroxide, calcium fluoride, talc, iron oxide, mica, iron sulfide, metal powders and vermiculite. Such other inorganic fillers may be used alone or as combinations of two or more thereof. The content of these other inorganic fillers in the friction material composition is preferably 0.1 to 70% by volume, more preferably 3 to 50% by volume, and most preferably 5 to 30% by volume.
Along with the inorganic filler having the specific 90% particle size and Mohs hardness noted above, the friction material of the invention preferably includes also a specific amount of glass chopped strand as the fibrous base.
It is preferred that the glass chopped strand have a fiber length of 2 to 5 mm, especially 2.5 to 3.5 mm, a fiber diameter of 5 to 12 xcexcm, especially 7 to 11 xcexcm, and a number of fibers per strand of 50 to 500, especially 100 to 400.
The content of glass chopped strand is preferably 1 to 6% by volume, more preferably 2 to 6% by volume, and most preferably 2 to 4% by volume, based on the overall composition. The addition of too little glass chopped strand results in low braking effectiveness and poor reinforcing effects, leading to cracking and fissuring. On the other hand, too much chopped strand increases the speed spread.
In addition to glass chopped strand, the friction material of the invention may contain also other fibrous bases commonly used in friction materials. Illustrative examples include inorganic fibers such as metal fibers (e.g., iron, copper, brass, bronze, aluminum), ceramic fibers, potassium titanate fibers, carbon fibers, rock wool, wollastonite, sepiolite, attapulgite and artificial mineral fibers; and organic fibers such as aramid fibers, cellulose pulp, aramid pulp and acrylic fibers. Any one or combinations of two or more thereof may be used. Such fibrous bases may be used in the form of staple fibers or powder. The content of these other fibrous bases is preferably 5 to 30% by volume, and more preferably 5 to 15% by volume, based on the overall friction material composition.
Examples of suitable organic fillers that may be used in the invention include cashew dust, reclaimed dire dust, rubber dust, graphite, nitrile rubber dust (vulcanizate) and acrylic rubber dust (vulcanizate). These may be used alone or as combinations of two or more thereof. The organic filler is added in an amount of preferably 0.5 to 60% by volume, and especially 5 to 35% by volume, based on the overall friction material composition.
The binder used in the composition may be any known binder commonly used in friction materials. Suitable examples include phenolic resin, melamine resin, epoxy resin, various rubber-modified phenolic resins, nitrile rubber, acrylic rubber and silicone rubber. These may be used alone or as combinations of two or more thereof. The binder is added in an amount of preferably 7 to 40% by volume, and especially 10 to 25% by volume.
The method of making the non-asbestos friction material of the invention involves first uniformly blending the above components in a suitable mixer such as a Henschel mixer, Redige mixer or Eirich mixer so as to give a molding powder, and preforming the powder in a mold. The preform is then molded at a temperature of 130 to 200xc2x0 C. and a pressure of 100 to 400 kg/cm2 for a period of 2 to 15 minutes.
The resulting molded article is postcured by heat-treating at 140 to 250xc2x0 C. for 2 to 48 hours, then cut, machined, ground and otherwise processed as needed to the required dimensions, giving the finished article.
In one embodiment of the invention, the friction material has a braking effectiveness TP1 at 5 km/h and a braking effectiveness TP2 at 30 km/h, as determined by low-temperature low-speed braking performance tests in accordance with Japan Automobile Technology Association standard JASO C407-87 (Dynamometer Test Methods for Truck and Bus Braking Equipment). The difference ratio between the braking effectiveness TP1 at 5 km/h and the braking effectiveness TP2 at 30 km/h, expressed as (TP1xe2x88x92TP2)/TP1xc3x97100, must be at most 40%, preferably at most 30%, more preferably at most 20%, and most preferably at most 15%. The lower limit in the difference ratio is not critical although a value of at least 0% is preferred. Too large a difference ratio may result in jerky low-speed braking, in which the braking effectiveness rises suddenly during brake operation at low speeds.
Typically, the low-temperature low-speed braking performance tests according to JASO C407-87 are conducted at a temperature of about 10xc2x0 C. The difference ratio (TP1xe2x88x92TP2)/TP1xc3x97100 as measured at 10xc2x0 C. is preferably at most 20%, and most preferably from 0 to 15%. The difference ratio (TP1xc3x97TP2)/TP1xc3x97100 as measured at 100xc2x0 C., which serves as the basis of comparison, is preferably at most 10%, and especially 0 to 7%.
In the low-temperature low-speed braking performance tests according to JASO C407-87, the braking torque T and brake fluid pressure P are measured at a simulated empty vehicle inertia and under four sets of conditions: a pre-braking brake temperature of 10xc2x0 C. or 100xc2x0 C. (basis of comparison) paired with an initial braking speed of 5 km/h or 30 km/h. These results are used to calculate the braking effectiveness, defined here as T/P, which is used in turn to calculate the rise in braking effectiveness at low speeds (i.e., the difference in effectiveness, given by T/P at 5 km/h-T/P at 30 km/h) and the difference ratio in effectiveness. The brake temperature was measured with a temperature sensor mounted on the brake assembly.
Preferably, the friction materials of the invention also have a change over time in speed spread, represented by the formula shown below, from a first (before bedding-in) effectiveness test to a final (fifth) effectiveness test in normal-use braking performance tests conducted at a simulated constant load inertia in accordance with JASO C407-87 (Dynamometer Test Methods for Truck and Bus Braking Equipment) of from 90 to 150%, and especially 90 to 120%. In addition, the change over time in braking effectiveness, represented by the formula shown below, is preferably 90 to 110%, especially 95 to 105%, at 50 km/h, and preferably 90 to 105%, especially 97 to 103%, at 100 km/h.
Percent change over time in speed spread=(speed spread in fifth test)/(speed spread in first test)xc3x97100
Percent change over time in effectiveness=(T/P at 50 or 100 km/h in fifth test)/(T/P at 50 or 100 km/h in first test)xc3x97100
After the first (before bedding-in) to final (fifth) effectiveness tests in normal-use braking performance tests conducted at a constant load inertia in accordance with JASO C407-87, the drum surface roughness, expressed as the average for ten points (RzD) in the roughness component in accordance with German standard DIN-4769, is preferably not more than 15 xcexcm, more preferably not more than 10 xcexcm, and most preferably from 1 to 7 xcexcm. The drum wear depth, expressed as the average for ten points in the depth of wear from the surface of a new drum, is preferably not more than 20 xcexcm, more preferably not more than 15 xcexcm, and most preferably from 5 to 13 xcexcm.
In constant load inertia normal-use braking performance tests according to JASO C407-87, the brake lining is mounted on a large truck rear wheel brake assembly. Using a brake dynamometer, the braking torque T and the brake fluid pressure P are measured by carrying out braking performance tests from a first (before bedding-in) to a final (fifth) effectiveness test under a simulated constant load (gross vehicle weight, 20 metric tons) in accordance with JASO C407-87. The resulting data are used to calculate the braking effectiveness T/P.
Using the friction materials of the invention, an automotive brake shoe assembly may be produced by placing the finished article on a steel or cast iron brake shoe plate that has been cleaned, surface treated and coated with an adhesive. The brake shoe and the finished article are then held together in this state under pressure and bonded by the application of heat. A brake shoe assembly for a bus or truck may be produced by riveting the finished article to a steel or cast iron brake shoe plate that has been cleaned and surface treated.
The non-asbestos friction materials of the invention are highly suitable for use in automobiles. Particularly when the friction materials are employed for large vehicles such as buses and trucks, the jerkiness during low-speed braking that has hitherto been a problem does not arise. The friction materials of the invention also lend themselves well to use in related applications, such as disc pads, brake shoes and brake linings in disc brakes and drum brakes.